Retention of Heavy Metals by Carboxyl Functional Groups of Biochars in Small Arms Range Soil

Abstract

Long-term effectiveness of biochar for heavy metal stabilization depends upon biochar's sorptive property and recalcitrance in soil. To understand the role of carboxyl functional groups on heavy metal stabilization, cottonseed hull biochar and flax shive steam-activated biochar having a low O/C ratio (0.04-0.06) and high fixed carbon content (~80% dry weight basis) were oxidized using concentrated H(2)SO(4)/HNO(3) and 30% HNO(3). Oxidized and unoxidized biochars were characterized for O/C ratio, total acidity, pH, moisture, ash, volatile matter, and fixed carbon contents, Brunauer-Emmett-Teller surface area, and attenuated total reflectance Fourier transform infrared spectral features. Characterized biochars were amended (2%, 5%, 10%, and 20% in grams of biochar per gram of soil) on a sandy, slightly acidic (pH 6.27) heavy metal contaminated small arms range soil fraction (<250 μm) having low total organic carbon (0.518%) and low cation exchange capacity (0.95 cmol(c) kg(-1)). Oxidized biochars rich in carboxyl functional groups exhibited significantly greater Pb, Cu, and Zn stabilization ability compared to unoxidized biochars, especially in pH 4.9 acetate buffer (standard solution for the toxicity characteristic leaching procedure). Oppositely, only oxidized biochars caused desorption of Sb, indicating a counteracting impact of carboxyl functional groups on the solubility of anions and cations. The results suggested that appropriate selection of biochar oxidant will produce recalcitrant biochars rich in carboxyl functional groups for a long-term heavy metal stabilization strategy in contaminated soils.

Effect of Biochars from Rice Husk, Bran, and Straw on Heavy Metal Uptake by Pot-Grown Wheat Seedling in a Historically Contaminated Soil

“…In addition to increasing soil pH and developing phosphate precipitates, surface adsorption by biochar was also thought to be one of the mechanisms for stabilizing Cd, Zn, and Pb in soil. Biochar normally has a large specific surface area due to its porosity and abundant functional groups, which are helpful in the adsorption of metallic elements (Uchimiya et al 2011;Uchimiya et al 2012). A general decrease in NH 4 NO 3 -extractable Cd, Zn, and Pb concentrations in soil occurred in fine biochar treatments, rather than in coarse treatments, due to higher specific surface area in fine biochar compared with the corresponding coarse biochar (Fig.…”

“…In addition to mitigation of global warming (Molina et al 2009), improvement of soil fertilities (Fellet et al 2011), enhancement of plant growth , and the increase of the nutrient retention capacity of soil (Laird et al 2010a), biochar is also drawing increasing interest for its ability to remediate organic and inorganic contaminants (Chai et al 2012;Uchimiya et al 2012).…”

“…Concentrations of Cd and Zn in soil pore water decreased significantly, whereas that of As increased markedly in a multi-element polluted soil amended with biochar under various soil moisture conditions (Beesley et al 2010). Biochar-induced increases in soil pH, the number of oxygen functional groups, and/or phosphorous levels caused the formation of ligands, which contribute greatly to the immobilization of toxic metals in soils and may result in As activation (Cao et al 2011;Uchimiya et al 2010;Uchimiya et al 2012;Sadiq 1997). In addition to changing the mobility of metal(loid)s in soils, the blocking capacity of iron plaque, root-to-shoot translocation in the plant, and the accumulation of metal(loid)s in rice shoot were influenced by biochar amendments .…”

Assessment of the Binding of Protons, Al and Fe to Biochar at Different pH Values and Soluble Metal Concentrations

“…Biochar has received considerable interest as a low cost and sustainable biosorbent to remove metal contamination such as As, Cd, Pb, Zn from waste water or acid mine drainage water [10][11][12][13][14][15]. Metal adsorption efficiency varied, however, and is affected by factors, such as functional groups, surface area, and environmental conditions [10,16,17]. Metal retention on organic materials is strongly pH dependent [18,19].…”

Is sustainability certification for biochar the answer to environmental risks?

“…Controls are required to ensure that biochar applied to soil does not exceed acceptable thresholds for heavy metals and other possible contaminants. Biochar application to contaminated soils can, in fact, reduce the availability of some heavy metals (Namgay et al, 2010;Uchimiya et al, 2012) and PAH (Gomez-Eyles et al, 2011).…”